Apparatus and method for magnetic recording



April 22, 1947. 5, J BEGUM 2,419,195

APPARATUS AND METHOD FOR MAGNETIC RECORDING Filed June 16, 1944 Patented Apr. 22, 1947 UNITED STATES PATENT OFFICE APPARATUS AND METHOD FOR MAGNETIC RECORDING Application June 16, 1944, Serial No. 540,667

12 Claims. I

This application is a continuation-in-part of application Serial No. 399,909, illed June 26, 1941.

This invention relates to magnetic recording and reproducing. Among the objects of the invention is a novel magnetic recording and reproducing system utilizing signal-modulated carrier frequency oscillations for recording signals in such a manner as to overcome difliculties and limitations of prior recording systems.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof, reference being had to the accompanying drawings wherein Fig. 1 illustrates a hysteresis loop for a magnetic material;

Fig. 2 is a diagrammatic illustration of obliterating and recording heads;

Fig. 3 is a diagrammatic illustration of a carrier current;

Fig. 4 illustrates a magnetization curve for a magnetic material;

Fig. 5 is a diagrammatic illustration of a modulated carrier current and a magnetization curve;

Fig. 6 is another diagrammatic illustration of a modulated carrier current;

Fig. 7 is a diagrammatic illustration of a device for normalizing a magnetic material; and

Fig. 8 is a circuit diagram illustrating a magnetic recording and reproducing system based on the principles of the invention.

The process of magnetically recording a signal on a magnetizabl material involves the use of an electric signal current to produce a corresponding magnetic impression on the magnetic medium. The magnetic impression has a predetermined relationship to the signal being recorded and is translated by suitable magnetic transducer means into an electric signal which corrresponds to the original signal.

The magnetic medium may be a tape or sheet having a magnetic surface layer of a thickness small with respect to its width, or it may be a wire or filament having a magnetic surface layer. Some features of the invention will be described in their application to an endless magnetic tape medium, but it is to be understood that the principles of the invention may be applied to all types of magnetic material such as wires, sheets, or cylinders.

In recording on a magnetic tape, an electric current is used to set up a. magnetizing force which is proportional to the current. Therefore, as the electric current varies in accordance with the instantaneous value of the signal to be recorded, the magnetizing force impressed on the tape is proportional to the original signal. This magnetizing force which is created proportional to the signal current is measured in gilberts per cm. and is indicated by the symbol H and the flux density induced in the tape by the magnetizing iorce H is indicated by the symbol "B measured in gauss.

The relationship between the magnetizing force H and the resultant magnetic induction 13" for increasing and decreasing values of H is expressed graphically by a hysteresis loop, and for increasing values of H starting with a demagnetized tape it is expressed by a virgin magnetization curve. Every magnetic material has a particular virgin magnetization curve and hysteresis loop which identifies the magnetic characteristic of the material. Materials such as a steel tape formed by a rolling process will have difi'erent magnetization curves for different directions of magnetization, and they may also have difierent magnetization curves at different points for the same direction of magnetization accordingly, the hysteresis loop illustrated by the heavy lines in Figure 1 is intended to be representative of the general shape of the magnetic characteristics of such materials.

In Figure 1; assuming that the magnetic material is originally in an unmagnetized condition, the branch or the magnetizing curve represented by the heavy line It shows the relationship between the magnetizing force 11" and the resulting magnetic induction 3" for increasing values of H. The curve starts at point 0, and after a short curve portion is substantially straight from about point C; to about point C: where magnetic saturation of the magnetic material starts, and is then curved to point S1 where saturation is substantially complete. When the magnetizing force is reduced the relation of induction B to force H does not follow back down along the curve It. but follows a new curve ll. When sufllcient reversed magnetizing force is applied to the material it again becomes saturated at point S2, but in this cas with opposite magnetic polarity. By again reversing the direction of the magnetizing force to its original polarity, a. curve I2 is traced from saturation point S2 to saturation point S1. Further reversals of the magnetizing force will cause the resulting magnetic induction to follow curves Ii and I2. Thes two curves constitute a hysteresis loop. Starting from point S1, if the maximum magnetizing force is gradually reduced as the direction of the field is continually reversed, the resulting magnetic induction in the material will follow a curve represented by the light lines [3, l4, l5, l6, l1, I8, and when the magnetizing force H reaches a value of zero the induction B will also have a value of substantially zero. In other words, the magnetic material will have no residual magnetism. From the hysteresis loops illustrated in Figure 1 it is obvious that magnetic effects are not reversible, and that the magnetic induction obtained from a given magnetizing force depends upon the previous magnetic history of the material on which the magnetizing force is impressed. Accordingly, with a tape upon which previous magnetic records have been made it is necessary to produce a substantially uniform condition either of saturation or demagnetization, in order to erase the magnetic record.

In a magnetic tape it is necessary to obliterate the previous record before a new record is put on the tape or mixed records are apt to result. This obliteration may be obtained by applying a magnetizing force H to the tape which is sufficiently high in value to cause an induction 13" which is in the saturation range on the magnetization loop of the material. That is-in the neighborhood of point S1 or S2 in Figure 1. It may also be obtained by applying over a relatively large area of the moving tape an alternating magnetizing force derived from an alternat ing current, and gradually reducing the magnetizing force to zero to reduce the residual mag netism to substantially zero. The relationship between "H and B for this decreasing alternating force is illustrated by lines l3, H, l5, l6, l1, l8 of Figure l, and subsequent magnetization in accordance with a signal will have a BH relationship in accordance with the virgin magnetization curve of the material. The decrease in strength of the magnetizing force is obtained for each portion of the tape by removing that portion of the tape farther and farther from the center of the magnetizing force H." Figure 2 illustrates diagrammatically a portion of a recording device comprising an endless tape 2| which is continuously driven past the obliterating head. and also illustrates the means for reducing the magnetic force on the tape. Continuous alternating current is applied to the coils 22 of obliterating pole pieces 23 which are positioned near the moving tape. This causes a rapidly reversing diffuse magnetic flux to be impressed through an area of the tape 2| which is large in comparison to the area through which the recording flux is impressed, and as the tape moves away from the pole pieces 23 which are the center of the flux distribution, the strength of the field on each portion of the tape decreases to zero. The pole pieces 23 of the obliterating head have their like magnetic poles toward each other as may be seen in Figure 2. However, the use of alternating current in the coils 22 causes the magnetic polarity of each pole 23 to alternate; at one instant north poles are facing each other, the next instant south poles are facing each other. This establishes a magnetic field about each pole pieces 23 which bucks the other magnetic field and results in a relatively large or diffuse magnetic field through which the tape 21 moves during the obliterating process.

The pole pieces 26, 21 of the recording head 25 have their unlike magnetic poles toward each other. The use of alternating current in the coils 28 which surround the pole pieces 28, 21 causes the magnetic polarity of each pole to alternate; at one instant the north magnetic pole of the pole piece 26 faces the south magnetic pole of pole piece 21, and the next instant the south magnetic pole of pole piece 26 faces the north magnetic pole of pole piece 21. This establishes aiding magnetic fields about each pole piece 26, 21 and results in a. relatively small or concentrated magnetic field having what may be called a focusing eilect through which the tape 2| moves during the recording process.

Each portion of the tape which has been removed sufliciently far from the obliterating poles 23 is now reduzed to substantially zero residual magnetism and is ready to have a magnetizing force corresponding to a signal impressed on it. This is done by the recording head indicated generally by the reference character 25 and comprised of two pole pieces 26, 21, each surrounded by a coil 28. In each coil 28 there is a current corresponding to the signal to be recorded on the tape. The current establishes a varying magnetizing force H" in the pole pieces 26, 21 which in turn establish a varying magnetic flux density B in portions of the tape 2|. In Figure 2 the pole pieces 26 and 21 are shown offset with respect to each other in position to cause the flux leaving pole piece 26 and entering pole piece 21 to travel for a short distance in the tape in a direction substantially parallel to the direction of movement of the tape. This is called the longitudinal method of magnetization. It is to be understood, however, that my method of magnetic recording may also be used with perpendicular recording in which the recording pole pieces are positioned in line with each other and on opposite sides of the tape to cause flux to pass through the tape in a direction substantially perpendicular to the direction of movement of the tape.

Recording a signal on a tape introduces distortion due to non-linearity of portions of the magnetization curve of the tape material unless steps are taken to cause the recording to be only on the linear portions of the magnetization curve. I effect recording on the linear portions of the magnetization curve of the tape material by applying to the coils 28 a current comprised of a carrier current modulated by a signal to be recorded. Any of the available methods for modulating a carrier current, such as amplitude modulation, frequency modulation or phase modulation, may be used for practicing the present invention. For the sake of simplicity, the form of invention shown herein uses the method of amplitude modulation.

Figure 3 illustrates a carrier current 30 the amplitude of which is modulated by a signal current to establish an envelope 3| which varies with the signal to be recorded. The carrier current frequency may be in the neighborhood of two or more times the frequency of the highest component of the signal. I have found that for most carrier frequencies the same oscillator can be used as a source of both carrier and obliterating current. This eliminates the need for an extra oscillator and makes a more simple device. However, if the need arises a carrier current of a frequency different from twice the frequency of the highest component of the signal to be recorded can be used. Signals having a frequency as low as zero (direct current signal) may be recorded. It is also to be understood that the obliterating current and the carrier current may be produced by separate sources (such as oscillators) and that the obliterating current frequency may be of any value which will give a sumcient number of reversals of polarity to reduce the residual magnetism in the tape to substantially zero.

The modulated carrier current in the coils 23 of the recording head 25 produces a, variable magnetiaing force H" in the pole pieces 26, 21, and produces in the previously demagnetized portions of the tape 2| which pass between the pole pieces a variable magnetic induction-B corresponding to the modulated carrier current. For good resuits the length of the focal area through the tape 2| should be smaller than the length of the shortest wave of the signal to be recorded. As the portions of the tape 2| which pass between the pole pieces 26 and 21 have been previously substantially demagnetized, the maximum magnetic induction in the tape for each carrier current cycle is in accordance with the magnetization curve of the tape material. By this technique the carrier oscillations are reproducibly recorded.

Figure 4 illustrates a representative magnetization curve for a material which may be used'for a magnetic tape. The curve comprises two substantially straight portions 32 and 33 and three curved portions 34, 35, and 36. The curved portion 36 lies between the two straight portions 32 and 33 and represents non-linearity between the magnetizing force and the induced flux density for small values of the magnetizing force. From point 31, which is substantially the junction of curved line 36 with line 32, to point 38, which is substantially the junction of line 32 with curved line 34, the line 32 is substantially straight. This means that for each unit increase in magnetizing force "H" between points 31' and 38 there is substantially a constant linear increase in the flux density B induced in the tape 2| fro1n3l" to 33". Above point 38 partial saturation takes place and B" does not increase in a constant ratio with H. Below the H axis the same effects take place for magnetizing forces created by currents of the opposite polarity. Point 39 represents the junction of lines 36 and 33; and point 40 represents the junction between line 33 and 35. Points 31, 38, and 39, 46 may be referred to as critical points as they define the extremities of the straight portions of the magnetization curve.

In order that a signal recorded on the tape 2| can be reproduced without considerable distortion it is necessary that the maximum magnetic flux induced in the tape during each cycle of the modulated carrier current correspond to a point which lies on the straight portion 32 or 33 of the magnetization curve of the material.

Figure 5 illustrates diagrammatically how the magnetizing forces H corresponding to a modulated carrier current induce maximum magnetic flux densities in the tape 2| which correspond to it is to be understood that iii) 6 36 of the magnetization curve of the material. This may be referred to as recording on the straight portions of the curve. In recording on a substantially demagnetized tape by my modulated carrier current system, the greatest dynamic range can be obtained by adjusting the normal amplitude of the unmodulated carrier current to have a peak value which corresponds to a magnetizing force "H" having a value which lies substantially midway between the value of the forces corresponding to points 31 and 33. The value of this force is represented by the reference character 45. Point 46 represents the value of this force for the opposite polarity of the carrier current. The minimum amount of distortion due to recording on the curved portion of the magnetization curve is obtained by adjusting the percentage of modulation of the carrier current to a value which is at all times below that is, with the normal peak value of the carrier current adjusted substantially midway between points 31' and 33, the peak value of the magnetizing force produced by any (each) cycle of the modulated carrier current will not be greater than the value of the magnetization force H represented by the point 38', nor less than the value of the magnetization force I-l" represented by the point 31. I have found that 50 to 60 per cent modulation is satisfactory for some magnetic materials. By this process the maximum values of the flux densities in the tape for each carrier cycle correspond .to points which lie only on the straight portions of the magnetization curve of the material, and therefore distortions of the signal reproduced from the envelope of the modulated'carrier is reduced to a minimum. The magnetizing force H corresponding to the amplitude of the peak of each cycle of the modulated carrier current is always less than the value of points 38', 40', to prevent recording on the curved portions 34, 35 of the magnetization curve, and is always greater than the value of points 31', 33' to prevent recording on the curved portion 36 of the magnetization curve. Depending upon the material used, the size of the curved portions 34, 35, and 36 varies. For each kind of material it is possible to adjust the amplitude of the unmodulated carrier current to cause the normal peak value of each cycle to fall substantially half way between points 31' and 33, and to adjust the amount or percentage of modulation of the carrier current by the signal to be recorded to cause the peak value of each cycle of the modulated carrier current to lie between points 31' and 33' for one polarity of the carrier current, and between points 33' and 40 for the other polarity of the carrier current.

For some types of recording it is possible that the envelope may be unsymmetrical with respect to the normal peak value of the carrier current. This is illustrated diagrammatically in Figure 6. In recording transients and male voices this unsymmetry is apt to be present. Depending upon the amount of unsymmetry which is expected it is possible to adjust the unmodulated carrier cur rent to cause its normal peak value to fall to one side of the points 45. 46 which represent the optimum peak value for the carrier current for symmetrical recording, thereby increasing the amplitude of the signal which may be recorded without distortion.

In some applications technical difficulties may prevent the use of the entire length of the straight portions 32, 33 of the magnetization curve. If such is the case the unmodulated carrier current may be adjusted to cause the peak value to fall to one side of the mid-points 45, 45.

In order to reduce magnetic coupling between closel positioned obliterating coils 22 and recording head 25, I position the obliterating coils symmetrically with respect to the recording head 25 and at substantially a 90 degree angle thereto. If it is found convenient to have the obliterating coils 22 in a position which tends to produce magnetic coupling with the recording head 25, a shield may be placed between the two heads thereby reducing the coupling,

In order to establish a relatively large diffuse magnetic field for demagnetizing the tape 2|, the coils 22 are positioned with their like magnetic poles toward each other.

The width of the tape 2| may be the same as the width of the recording and playback pole pieces as is illustrated in Figure 2, or the tape 2| may be narrower or wider than the width of the pole pieces 26, 21. Figure '7 illustrates a tape 20 which is wider than the width of the pole pieces 26, 21 and which embodies advantages over tapes which are not wider than the pole pieces. If the tape on which the signal is to be recorded is of the same width or narrower than the recording pole pieces any slight lateral change in the position of the tape with respect to the pole pieces will introduce a loss in flux through the tape or a magnetization pattern which in plan is not straight and which is apt to not reproduce well. The wide tape guards against these disadvantages as considerable lateral change in position will not materially affect the quality of the reproduced signal.

I have found that the transient in the reproduced signal caused by the joint in an endless tape is much less objectionable with my recording process using a modulated carrier current recording on a substantially demagnetized tape than for processes using a polarizing current. This may be for the reason that an unmagnetized tape joint does not cause a transient and the change of the tape joint to be magnetized to any extent in m device is much less than in the devices using a polarizing current. According to my invention the tape joint will be strongly magnetized only when there is a strong modulated signal on the portion of the tape occupied by the tape joint, and will be much less magnetized when there is little magnetization on the tape. It will be seen, therefore, that the average distortion caused by a tape joint magnetized according to the method of my invention will be less than the average distortion caused by a tape joint magnetized according to the polarizing method.

Fig. 8 is a diagram indicating the general arrangement of one form of magnetic recording and reproducing system based on the principles of the invention. An electric signal input source, such as a microphone impresses the input signal voltage on a recording circuit system 52, which, in turn, impresses the electric signal-modulated carrier-frequency oscillations on a magnetic transducer structure 53 which is magnetically interlinked with a moving element of a magnetic signal carrier track 54 for effecting corresponding variations of its magnetic characteristics and forming thereon corresponding magnetic records.

Either amplitude modulation or frequency modulation or phase modulation, for instance, of the general type described in the Radio Engineers Handbook" by F. E. Terman, published in 1943,

pages 531-588, 621,673, may be used for providing the signal-modulated carrier-frequency oscillations of a frequency range which is reproducibly recordable on a magnetic recording medium, and the carrier frequency oscillator circuits with the associated modulator and amplifier circuits are suitably combined in the recording circuit system 52 shown.

As indicated in Fig. 8, an obliterating structure 55 for obliterating each element of the moving magnetic signal carrier before it reaches the transducer head structure is shown connected to the recording circuit system for impressing thereon the carrier frequency obliterating currents. In the reproducing process, the remanent magneto-motive forces of the elemental magnets forming the magnetic waves recorded on the recording medium induce in the windings of the magnetic transducer structure 53 corresponding voltages which are impressed on a reproducing circuit system 51. The interconnections of the signal transducer structure 53 are shown controlled by a multi-blade switch 56 which, when actuated from the position shown to the opposite position, de-energizes the obliterating head 55 and connects the windings of the transducer structure 53 to the reproducing circuit system 51 instead of to the recordin circuit system 52. The reproducing circuit system 51 combines the demodulation and detecting circuits as well as the band-pass and low-pass filter circuits, for instance, of the general type described in the Radio Engineers Handbook, for impressing on a reproducing device, such as a loudspeaker 58, output corresponding to the input signals impressed by the input source 5| on the recording circuit system.

In designing such signal modulated carrier frequency magnetic recording systems, it is important that the speed of the signal track relatively to the transducer head and the magnetic slit thereof be so chosen and correlated as to assure that for the highest side band frequencies of the carrier used in recording, the magnetic record track of such frequencies shall have wave lengths greater than the magnetic recording slit or, in general, the effective magnetic gap between the pole pieces of the transducer head.

The extent to which the wave length of the recorded carrier may approach the recording slit or pole piece gap with good recording and reproducing depends on the coercive force and other characteristics of the recording medium. If a recording medium of high coercive force is used, higher carrier frequencies may be used in the recording. In general, the carrier frequency should be about three times higher than the highest modulation frequency. Thus, for instance, for making records of telephone conversations or of dictation requiring good reproduction over the speech frequency range up to about 2500 cycles, the carrier frequency should be of the order of about 10,000 cycles.

The magnetic cores of Fig. 2 are only diagrammatically illustrated. It is understood, however, that any practical magnetic core construction may be used as for example those described and illustrated in applicant's oo-pending application Serial No. 612,728, filed Aug. 27, 1945, as a continuation-in-part of the instant application.

The features and principles underlying the invention described above in connection with specific exemplifications, will suggest to those skilled in the art many other modifications thereof. It

.9 is accordingly desired that the appended claims be construed broadly and that they shall not be limited to the specific details shown and described in connection with exempllilcations thereof.

I claim:

1. The process of recording a signal on magnetic material, comprising the steps of: providing a carrier current having a normal peak amplitude, said current being adapted to establish for the normal peak amplitude a magnetizing force in the said material corresponding to a point on the magnetization curve of the material substantially mid-way between upper and lower critical zones, modulating said carrier current by a signal to establish two envelopes, and applying to the material magnetizing forces having peak values corresponding to both envelopes of the modulated carrier current, the modulation of said carrier current being adjusted to cause the flux density in the material corresponding to the peak value of each one-half cycle of the modulated carrier current to correspond to a point which lies on the magnetization curve of the material between said upper and lower critical zones.

2. The process of magnetically recording a signal on magnetic material comprising the steps of: providing-a carrier current, modulating said carrier current by the signal to be recorded to establish two envelopes, and applying to said magnetic material magnetizing forces having successive peak values corresponding to both envelopes of said modulated carrier current, the modulation of said carrier current being at all times below 100 per cent, and the peak flux density induced in the material by said magnetizing forces corresponding to both envelopes being substantially less than saturation.

3. Th process of magnetically recording a signal on a moving magnetic member, comprising the steps of: reducing the residual magnetism in successive incremental portions of the magnetic member to substantially zero, providing a carrier current, modulating the carrier current by the signal to be recorded to establish a first and a second envelope, and applying to the successive incremental portions of the magnetic member magnetizing forces corresponding to successive values of both the first and second envelopes of the modulated carrier current, the flux density induced in the magnetic material by the magnetizing forces corresponding to both said first and said second envelopes being at all times substantially less than saturation.

4. The process of magnetically recording a signal on a material, comprising the steps of: providing an alternating carrier current having a normal peak amplitude, modulating said carrier current by the signal to be recorded, applying to the material a varying alternating magnetizing force having two polarities corresponding to the modulated carrier current and adapted to establish a magnetic flux in the magnetic material which has a value which lies on both sides of the zero point of the magnetization curve of the material, and adjusting the normal peak amplitude of the carrier current and the modulation of said amplitude by the signal to be recorded to cause the maximum flux density resulting in the material from each cycle of the magnetizing force to lie below the saturation range of the material for both polarities of the magnetizing force.

5. The process as set forth in claim 4 in which the signal to be recorded is a direct current signal.

6. The process of magnetically recording a signal on a moving magnetic member comprising the steps of: providing a magnetic material having substantially no magnetization thereon, providing a. carrier current having a frequency which is within the recordable range, modulating the carrier current by the signal to be recorded to establish a first and a second envelope, and applying to the successive incremental portions of the magnetic member magnetizing forces corresponding to successive values of both the .lrst and second envelopes of the modulated carrier current, and the flux density induced in the magnetic material by the magnetizing forces corresponding to both said first and said second envelopes being at all times substantially less than saturation.

7. The process as set forth in claim 6 in which said signal to be recorded is a direct current signal.

8. The process of magnetically recording a signal on a material, comprising the steps of: providing an alternating carrier current having a normal peak amplitude and having a frequency which is within'the recordable range, modulating said carrier current by the signal to be recorded, recording said modulated carrier current by applying to the material a varying alternating magnetizing force having two polarities corresponding to the modulated carrier current and adapted to establish a magnetic flux in the material which has values which lie on both sides of the zero point of the magnetization curve of the material, and adjusting the modulation of said amplitude by the signal to be recorded to cause the maximum flux density resulting in the material from each cycle of the magnetizing force to lie below the saturation range of the material for both polarities of the magnetizing force.

9. The process of recording a signal on a magnetic material comprising the steps of: altering the magnetic state of said material to render the flux density in the material substantially zero, providing a carrier current having a frequency which is within the recordable range, modulating said carrier current by the signal to be recorded, and recording said modulated carrier current by passing said material through a magnetic field established in accordance with said modulated carrier current.

10. The method of magnetically recording a signal on magnetic material, that includes moving the material through a magnetic field produced by an alternating signal-modulated carrier current, the frequency of the signal and the frequency of the carrier being such that the upper side band is magnetically recorded on the magnetic material.

11. In a process of magnetically recording signals on successive elemental portions of a substantially demagnetized permanently magnetizable record track which moves at a substantially constant speed relatively to a magnetic recording head, the steps of producing in the recording head magnetic oscillations of a carrier frequency which are substantially reproducibly recordable on the magnetizable record track at the said constant speed; the carrier oscillations having an amplitude such that the unmodulated amplitude peaks of the carrier oscillations ma netize the record track along those zones of its magnetization characteristics in which the ma netic flux induced in the track varies with the magnetizing field in a generally straight line relationship; and modulating the carrier oscillations with a magnetic signal flux corresponding to the signal to be recorded: the signal amplitude SEMI JOSEPH BEGUN.

12 REFERENCES CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Camras June 13, 1944 Camras June 13, 1944 Camras June 13, 1944 Camras June 13, 1944 

